A. Field of the Invention
This invention relates to the formation of a fluid-tight seal at an aperture where a rotatable shaft passes through a barrier means into a barred fluid.
B. Prior Art
In rotatable shaft applications where the shaft passes through a barrier, such as rotatable shafts in pumps, fans, compressors, agitated tanks, boat propeller shafts and the like, it is often beneficial or necessary to seal the aperture where the shaft passes through the barrier into a barred fluid. For example, reactors, tanks and vessels handling toxic or corrosive chemicals are often agitated to mix and suspend solids. The chemical is the barred fluid in the tank which must be sealed from the atmosphere. Or, for example, the body of water through which a boat is propelled is the barred fluid which must be sealed from the propeller drive.
Radial and axial movement of the rotatable shaft relative to a stationary barrier impedes effective sealing. Misalignment of the shaft relative to the barrier and radial or axial movement of the barrier relative to an aligned shaft also impede effective sealing. Axial compression and extension of the shaft relative to the barrier are impediments to an effective seal as well. The nature of the barred fluid material must also be considered in designing a shaft seal.
Packing glands and mechanical seals are typically employed to seal rotatable shafts. A stationary portion of the gland or seal is attached to the barrier and a rotatable portion of the gland or seal is attached to the shaft and allowed to rotate with the shaft.
Packing glands provide a seal by radial expansion of a lubricated deformable material such as braided or layer-wound rubber, plastic, fiber or a combination of these. Radial expansion is accomplished by axially compressing rings made of the deformable material which are stacked against the rotatable shaft in a restricted space. The packing gland must be periodically lubricated. Because it is a rubbing contact seal, heat is generated by friction with the rotating shaft, and a cooling jacket surrounding the packed deformable material is often necessary. The utility of packing glands is limited by the type of deformable material used. The packing gland will leak or fail if the packing material corrodes, loses its compressibility, does not retain its integrity at high temperatures, is not resilient enough to withstand the operating deflections of the shaft, or is insufficiently lubricated. Packing glands are typically employed to seal boat propeller shafts.
A mechanical seal is not in rubbing contact with the shaft, but rather employs at least two flat ring-shaped surfaces: one mounted on the barrier in a stationary non-rotating position, and the other attached to and rotatable with the shaft. The two surfaces or faces are extremely close to each other and may have a thin film of lubricant between them or be self-lubricating (one surface may be carbon or Teflon, for instance). The faces are orientated at right angles to the axis of shaft rotation. The pressure forcing the faces together must be great enough to maintain their proximity yet not so great as to close the gap and prevent rotation. A typical size gap is on the order of 0.00001 inches. The gap must be kept free from all foreign material to maintain the gap size, prevent abrasion of the surfaces, and avoid leaking. If the gap is permitted to open to even 0.001 inches, abrasives may penetrate and leaking may occur.
As the rotatable face rotates with the shaft, any radial or axial movement of the shaft relative to the barrier forces the sealing faces apart. As the stationary face is fixed relative to the barrier any radial or axial movement of the barrier relative to the shaft also forces the sealing faces apart. For example, where a mechanical seal is used in connection with an agitator shaft rotatable in a tank or reaction vessel made of fiberglass reinforced plastic, movement of the tank nozzle where the shaft enters the tank can be extreme because the fiberglass reinforced plastic tank has a low modulus of elasticity and actually flexes or wobbles during agitation. In the manufacture of glass lined steel tanks or reaction vessels, the high firing temperature can warp the tank nozzle making it extremely difficult to center the shaft in the nozzle. The rotatable face of a mechanical seal attached to a misaligned shaft will be forced away from the stationary face attached to the misaligned tank nozzle. Accordingly, insulating the mechanical seal faces from the separating forces of radial and axial movement of the rotatable shaft and stationary barrier decreases the likelihood of leakage or seal failure.
Attempts to insulate the seal faces of mechanical seals from the separating forces of axial and radial movement of the shaft and barrier have focused on altering the dimensions of the mechanical seal components. Mechanical seals have been improved by increasing the diameter and surface area of the seal faces which increases tolerance for radial motion.
Another method of insulating the seal faces from separating forces is the use of a double seal comprising two mechanical seals installed along the axis of the rotable shaft. Use of a double seal increases tolerance for axial motion. In a double seal two mechanical seals are positioned so that their rotatable faces are simultaneously forced against their respective stationary faces by one or more compression springs which are capable of absorbing some axial motion.
In yet another attempt to increase mechanical seal tolerance a "floating stationary unit" is employed. In a floating stationary unit the non-rotatable seal faces of a double seal are not truly stationary, but rather are permitted to float relative to the barrier while remaining aligned with the rotatable faces when the shaft is subjected to radial motion.
These advances in the field have afforded only minimal increases in tolerance. An example of a mechanical seal employing a floating stationary unit and offering the maximum tolerance available in the industry is the "Chesterton 222" manufactured by A. W. Chesterton Company of Stoneham, Mass. The Chesterton 222 tolerates a maximum of one eighth inch radial motion of the shaft at the point where the seal is installed on the shaft.
As noted above, in the chemical industry corrosive materials must often be sealed from leaking to the atmosphere. A mechanical seal used for this purpose is often exposed to the corrosives and accordingly is made, in part, of expensive corrosion resistant metals or metal alloys such as nickel, Hastelloy-C or tantalum.
The novel apparatus and method of the present invention overcomes the deficiencies of the prior art by insulating the seal faces of a mechanical seal from the separating forces of axial and radial shaft and barrier motion, by protecting the mechanical seal faces from abrasives, by stabilizing the stationary portion of the mechanical seal relative to the rotatable shaft, by stabilizing the rotatable portion of the mechanical seal relative to the stationary portion and thus making the seal self-centering, and by permitting the use of less expensive mechanical seals made of, for example, stainless steel even in corrosive applications.
It is an object of the present invention to insulate the seal faces of a mechanical seal from the separating forces of axial and radial shaft and barrier motion without altering the means by which a rotatable shaft is attached to the rotatable portion of a conventional mechanical seal.
It is a further object of the present invention to provide a mechanical seal assembly having a tolerance for axial and radial motion greater than the tolerance of a conventional mechanical seal.
It is a further object of the present invention to protect the seal parts of a mechanical seal from corrosives, abrasives and foreign materials.
It is yet a further object of the present invention to stabilize a mechanical seal relative to the rotatable shaft.
It is yet a further object of the present invention to stabilize the rotatable portion of a mechanical seal relative to the stationary portion.
It is yet a further object of the present invention to minimize the accuracy required to properly mount a shaft drive means for a rotatable shaft relative to the barrier through which the rotatable shaft passes by making the seal means self-centering.
It is yet a further object of the present invention to permit use of mechanical seals made of inexpensive non-corrosion resistant materials in corrosive applications.
These and other objects of the present invention will become apparent to those skilled in the art from the following description and accompanying drawings of the present invention.